Android 5 may have improved security on newer phones running it but the performance issues and the new interface are what drive the customers away when it initially launched. There are still people using IOS 8 and Android 5 of course but the amount of customers using Android 5 is unspeakably low compared to those using Android 4.x. Maybe, those superior Android devices are super-expensive and especially for those running the later versions of Android OSes. Another problem is that your current Android device cannot be upgraded to Android 5 if it doesn’t support that.
Then, for Android M, it has better security than ever with the opportunity for the users to accept or deny the app permissions just like what Cyangenmod OS has. Other than better security, there’s also the battery saving feature such as Doze in which when the device is inactive for a long period of time, the app activity will be inactive as well. For tablet devices, this feature may be handy like you can place it on the table when you’re not using it but I still think that it should be better in saving battery when the device is on standby with app and service activity inactive. The activity should be really inactive when the device is on standby for a long period of time but I don’t know if this is the point of that.
There are other features such as app auto-backup, USB Type-C support, new volume control, certificate installation for enterprises and so on. I don’t know how the app auto-backup works but I guess that this is Google’s official way of backing up your apps. I guess that the destination for that will be the cloud storage or something.
Android M will be released in the third quarter of this year.